The present invention relates to a signal meter circuit for an automatic tuner of a radio receiver.
FIG. 4 shows a conventional AM tuner having a signal meter circuit. An output of an AM front end 2 connected to an antenna 1 is connected to an intermediate-frequency amplifier 3. The output of the intermediate-frequency amplifier 3 is demodulated by a detector 4, which is outputted from an output terminal 9. An automatic gain control (AGC) 7 is provided between the output of the detector 4 and the intermediate-frequency amplifier 3 through one of low pass filters 5 and 6, which are alternatively selected by a switch 8. The low pass filters are provided for producing direct current components from the output of the detector 4. A signal meter 10 is connected to the output of the intermediate-frequency amplifier 3 in the loop of the AGC 7.
Namely, the output signal to the signal meter 10 is controlled by the AGC 7 to an approximately constant level. Consequently, the output signal does not linearly change with respect to the level of received energy. In other words, the output signal does not change largely with the change of the electric field strength, which causes error in tuning operation.
On the other hand, the low pass filter 5 has a time constant T.sub.1 having a longer time as several tens msec. for receiving waves for stabilizing the level of the output of the AGC loop. To the contrary, the low pass filter 6 has a shorter time constant T.sub.2 as several msec. for scanning the frequency band for the tuning. As a result, the switch 8 must be provided for selecting one of the LPFs 5 and 6. Accordingly, the circuit becomes complicated in construction.